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The focus ring on the Tamron is quite firm but smooth. As long as it's not knocked, it will hold the focus pretty well, no need to lock. They are a bit slow at f8 and I am forced to use higher ISO values to keep the exposure short for the little Nano Tracker. At 500mm focal length, 40 second subs is about the limit on this mount. There'd be no way I would use a 2X converter. It would be near impossible to get enough exposure at f16 let alone a focal length of 1000mm and keep the stars round on this mount!
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